Dockerfile nrUE ubuntu: install arping

The CN team would like to test the ethernet PDU session capabilities of
the UPF. Install arping in the UE by default for ubuntu to help them
test the PDU session by pinging over ARP.

docker/Dockerfile.nrUE.ubuntu

@@ -39,6 +39,7 @@ ENV TZ=Europe/Paris
 RUN apt-get update && \
     apt-get upgrade --yes && \
     apt-get install --yes \
+        arping \
         software-properties-common \
         procps \
         libsctp1 \